1. Field of the Invention
The present invention concerns tire wear control, and in particular, tires that allow automatic detection that a predetermined tire wear threshold has been reached.
2. Related Art
As used herein, the term “cord” is understood to mean a single thread or wire, metallic or non metallic, or an assembly of several threads or wires. “Ply” refers to a layer of parallel reinforcing cords, metallic or non metallic, woven or unwoven, and embedded in a rubber composition, the thickness of the ply being close to the diameter of an individual cord. “Belt” refers to an assembly of at least two plies that is unanchored to the bead. “Axial” refers to a direction essentially parallel to the rotation axis of a tire. “Radial” refers to a direction parallel to a vector perpendicular to the axial direction and which intersects the rotation axis of the tire. “Tire” refers to pneumatic or non pneumatic tires.
As is known, the tread of a tire undergoes wear that results from the rolling of the tire on the ground. If the tread has grooves, the effect of this wear is to reduce the depth of the grooves, thereby degrading the performance of the tire on wet ground. On reaching a certain wear level, referred to as “tread wear limit”, the tire no longer enables rolling to take place under optimum conditions. The tire or its tread (“retreading”) is then replaced and scrapped or, if the residual thickness of the tread so allows, the grooves of the tread are cut out afresh.
The wear level of a tread can be monitored by measuring the depth of the grooves, but that operation is often neglected because it is considered troublesome and because it requires a suitable measuring tool, which may not always be at hand. It has therefore long been proposed to position wear indicators on the rolling surfaces so as to visually alert the driver of the vehicle when the minimum tread groove depth has been reached. The prior art contains an almost innumerable variety of propositions to provide pieces of colored rubber within the tire tread which appear on the tread surface when a tire wear threshold is reached. Industrial applications of such colored wear indicators have nevertheless remained relatively rare, not least because their precise positioning within the tread is difficult to obtain and costly. More common visual tire wear indicators take advantage of the visual contrast between the rubber forming the tread and a groove or cavity in the the tread.
Irrespective of their precise nature, visual tire wear indicators have the drawback that the user has to has to periodically inspect the tires in order to realize when the wear limit is reached. It has therefore been proposed (see, for example, US 2002/0036039) to monitor the tread wear, as revealed by such wear indicators, by image analysis, which can be used to generate an alert signal as soon as the wear limit is reached.
Another, more direct way of obtaining information on the tread wear is to use magnetic or electrical sensors whose signals are altered as a function of tread wear.
There have been several proposals to use magnetic properties of the tread itself in order to detect the progress of wear. For example, DE 199 54 825 divulges the use of tread blocks made from magnetic rubber, the presence (or absence) of which is detected. DE 199 57 645 extends this concept to providing areas of alternating magnetic polarity in the depth of the tread.
Another approach is to provide conductors inside the tread and to detect changes of measurable parameters related to the removal of these conductors by wear. DE 25 24 463 divulges the idea of incorporating an electrical conductor into the tread, such that the conductor is cut as soon as the wear limit is reached. The same concept has later been proposed by JP 61-150 804.
DE 197 45 734 describes the use of a sensor comprising a number of conductive loops which extend at different heights in the tread and open successively as tread wear progresses, so that they form open switches; a detection circuit delivers a corresponding signal to an assessment unit present in the vehicle.
DE 199 30 046 describes a tread containing a bundle of metallic cords located precisely at the minimum lawful profile depth and wearing away when the profile is worn below this depth. An induction current passing through the bundle changes as the diameter reduces and initiates an audible and/or visual warning.
WO 03/031208 describes a method consisting in measuring the electrical resistance or capacitance inside a tread block and deducing the height of the block therefrom.
US 2005/061069 teaches the incorporation into the tire of a resonant circuit comprising several capacitors connected in parallel by means of conductive branches which extend into the tread. When the tread is worn, the branches extending into the worn part break, which changes the tuning frequency of the resonant circuit.
The present invention aims at improving this concept by simplifying the underlying tire structure, taking advantage of the electromagnetic behavior of the tire's belt.